(a) Field of the Invention
The present invention relates to a variable valve timing system for a vehicle, and more particularly, to a variable valve timing system that can maximize engine output efficiency by adjusting an open/close timing of an intake valve according to an engine rpm.
(b) Description of the Related Art
As shown in FIG. 4, a valve operation mechanism for intake and exhaust valves for an engine includes an intake camshaft 101 and an exhaust camshaft 103 which are independently provided to open and close the respective intake and exhaust valves in a timely fashion.
Furthermore, the exhaust camshaft 103 is driven by power transmitted from a crankshaft 105, and the intake camshaft 101 is driven by power transmitted from the exhaust camshaft 103 via a chain 107.
Here, the intake and exhaust valves are opened and closed in a timely fashion by the intake camshaft 101 and the exhaust camshaft 103. This is called valve timing, in which the intake valve is generally opened before an intake stroke (that is, before a top dead center point), and is closed after the intake stroke is completed (that is, after the bottom dead center point), and the exhaust valve is opened before an exhaust store (that is, before a bottom dead center point) and is closed just after the start point of the intake stroke after the finish point of the exhaust stroke (that is, after the top dead center point).
In addition, there is a valve overlap time between the finish point of the exhaust stroke and the start point of the intake stroke, during which both the intake and exhaust valves are open to completely exhaust the burned gas and increase the charging efficiency of the mixture.
However, in the conventional valve timing, although an overlap angle is uniform, the overlap time is reduced at a high rpm range of the engine, and is increased at a low rpm range. That is, the overlap time is varied according to the engine rpm. Particularly, in the high rpm range, since the overlap time is too short, the mixture being taken in cannot sufficiently expel the burned gas out of the combustion chamber. That is, the conventional valve operation mechanism cannot properly perform its function.
In addition, the chain is designed to be subject to vibration from the crankshaft and the camshaft to generate lateral vibration, causing impact noise with a chain guide and a sprocket.
The present invention has been made in an effort to solve the above problems.
It is an objective of the present invention to provide a variable valve timing system that can maximize engine output efficiency by properly adjusting an open/close timing of an intake valve according to an engine rpm.
It is another objective of the present invention to provided a variable valve timing system that can prevent impact noise between a chain and a chain guide by absorbing lateral vibration of the chain.
To achieve the above objectives, the present invention provides a variable valve timing system for a vehicle, comprising an intake camshaft for operating an intake valve, an exhaust camshaft for operating an exhaust valve, an intake cam sprocket mounted on the intake camshaft, an exhaust cam sprocket mounted on the exhaust camshaft, a chain interconnecting the intake cam sprocket and the exhaust cam sprocket, and means for varying phases of the intake and exhaust camshafts by, according to an engine rpm, pushing and pulling the chain to forcedly rotate the intake and exhaust cam sprockets at a predetermined angle while moving in a perpendicular direction with respect to a longitudinal direction of a power transmission member.
According to an embodiment of the present invention, the means for varying comprises an auxiliary cam sprocket disposed between the intake and exhaust cam sprockets and engaged with the chain, and means for moving the auxiliary cam sprocket in a perpendicular direction with respect to a longitudinal direction of the chain to push and pull the chain in the perpendicular direction, thereby varying the phase angles of the intake and exhaust camshafts.
Preferably, the means for moving comprises a slide bar for rotatably supporting the auxiliary sprocket, the slide bar being slidably disposed in a cavity formed in a cylinder head and defined upper and lower oil chambers with its ends against the cavity, each of the upper and lower oil chambers connected to an oil pump and an oil tank through intake and exhaust oil passages formed in the cylinder head, respectively, first and second check valves respectively disposed in the intake oil passages communicating with the upper and lower oil chambers, the check valves allowing the supply of oil from the oil pump to the oil chambers and disallowing the supply of oil from the oil chambers to the oil pump, first and second solenoid valves respectively disposed on the exhaust oil passages communicating with the upper and lower oil chambers, the first and second solenoid valves selectively opening and closing the exhaust oil passages, and an electronic control unit for controlling the first and second solenoid valves according to the engine rpm to selectively open and close the exhaust oil passages, thereby sliding the slide bar along the cavity.
The cavity is provided with at least one large diameter portion in which a projection formed on the slidebar is disposed to limit the slide movement of the slide bar to a predetermined length.
The slide bar is provided with a sprocket hole, the auxiliary chain sprocket is rotatably supported in the sprocket hole by a shaft, and a bearing is disposed between the shaft and the auxiliary chain sprocket.